Blacklist sphinx 2.1.0 (autodoc bug)
Blacklist sphinx 2.1.0 (autodoc bug)

See https://github.com/sphinx-doc/sphinx/issues/6440 for upstream details
Depend-On: https://review.opendev.org/#/c/663060/

Change-Id: I8751db5a1446be78b07608d39fb9c741bae78fde

openstack/python-muranoclient,openstack/python-muranoclient